This invention relates to an ultraviolet radiation starting source for an arc discharge lamp.
High pressure metal halide arc discharge lamps typically comprise an arc tube which encloses an ionizable fill material and two electrodes at opposing ends of the tube. To reduce the time it takes to start the lamp, a starter electrode may be disposed inside the arc tube near one of the main electrodes, as shown in Freese et. al., U.S. Pat. No. 3,900,761. A discharge can be initiated between the starter electrode and one of the main electrodes at a voltage that is much lower than the voltage required to ignite an arc between the two main electrodes. The ultraviolet radiation from this discharge produces photoelectrons which enhance gas breakdown and discharge formation in the arc tube between the two main electrodes.
Zaslavsky et. al., U.S. Pat. No. 4,818,915, issued Apr. 4, 1989, discloses a UV enhancer which is separate from the arc tube. The '915 patent, which is incorporated herein by reference, describes a UV enhancer which typically has a borosilicate glass envelope enclosing an ionizable fill material and a single electrode. The single electrode has a getter which can remove certain gases when the envelope heats and outgasses. When energized, the UV enhancer produces ultraviolet radiation which illuminates the path between the main electrodes within the arc tube, thus decreasing the time for generating a high intensity arc discharge.
The starter electrode approach and the separate UV enhancer each require additional parts and manufacturing steps. The extra parts and steps add to the lamp manufacturing cost.
It is an object of the present invention to provide an improved starting source for an arc discharge lamp.
It is another object of the present invention to provide a starting source which has fewer components than prior art devices.
It is yet another object of the present invention to provide a starting source which is easy to manufacture.
It is still another object of the present invention to provide a starting source which efficiently couples radiation to the interior of the arc tube.